This project supports studies of diffusion and convection in the extracellular spaces of cultured epithelial monolayers with the objective of improved understanding of the mechanisms for transepithelial solute and water transport. The measurements employ video microscopy with indicator dyes whose fluorescence properties can be rapidly altered by photobleaching or photolytic uncaging. The quantitative interpretation of the measurements is aided by mathematical modeling of the underlying physical and chemical processes. - epithelia, diffusion, water transport, video microscopy, photobleaching, uncaging, mathematical modeling